DIABROTICA Report Summary

Occurrence and abundance of indigenous natural enemies of D. virgifera

The aim of this subcontracted project was to monitor the occurrence and abundance of indigenous natural enemies of D. virgifera and of closely related species in their area of origin in Central and South America. Thus, field surveys were conducted to mass-collect adult Diabroticite beetles in Mexico, Argentina, and Brazil between 2000 and 2002. Adult beetles were reared to screen for natural enemies that could be used as potential biological control agents for control of D. virgifera in Europe.

In Mexico, nearly 10,000 beetles of seventeen species were routinely collected in the state of Veracruz, three of them from the virgifera group.

Two tachinid flies, Celatoria compressa Wulp and Celatoria bosqi Blanchard, as well as one braconid wasp, Centistes gasseni Shaw, were found and identified as parasitoids of adult Diabroticite beetles. Two other tachinid flies, Celatoria setosa (Coquillett),and Celatoria diabroticae (Shimer),and the braconid wasp, Centistes diabroticae (Gahan), were not studied because of their minor importance as potential biological control agents for the European situation. In Mexico, the adult parasitoid, Celatoria compressa was reared from 17 species in four different genera of Chrysomelidae between 2000 and 2002. In 2001, 2,436 beetles belonging to different species were collected at various locations in Veracruz, and 55 parasitoid pupae were obtained. In 2002, we collected 4,844 beetles at the same locations, and 99 tachinid puparia were found. This represents an overall parasitism of 2.3% and 2.0% respectively. In 2001, tachinid larvae emerged from six species in four genera, and in 2002 from seven species in the same four genera. Parasitism within host species ranged from 0.3% in Gynandrobrotica nigrofasciata (Say) to 16.7% in Cerotoma atrofasciata Jacoby. The most abundant species in Veracruz were Acalymma blomorum and Diabrotica balteata. The former suffered from 3.5% (2001) and 0.8% (2002) parasitism, and the latter from 4.6% parasitism in 2002.

In August 2001, we collected 995 beetles of three different species in the state of Oaxaca (Acalymma trivittatum, Diabrotica balteata, D. v. zeae) from squash  maize cultures, and obtained a total of 24 pupae, mainly from A. trivittatum (19), representing a 5.0% parasitism rate, and an overall parasitism rate of 2.9% for the three species at those locations In September 2002, we found only 585 beetles in the same locations, probably due to adverse environmental conditions (prolonged dry season followed by heavy rains). The parasitism rate was considerably lower, 0.8%, again mainly in A. trivittatum (1.9%)

In 2001 and 2002, populations of D. v. virgifera were collected in the state of Durango, northern Mexico. Beetles were found during the maize growing season from late August through October. Numbers of D. v. virgifera in maize monocultures in Durango were nevertheless surprisingly low and comparable to numbers of other Diabroticites collected from larger maize-squash fields in Veracruz. From a total of 640 D. v. virgifera collected in maize fields in Durango, 36 parasitoid pupae were collected in 2001, which represented a mean parasitism rate of 5.6% (max = 16%). In contrast, 304 D. v. virgifera were collected in the same region in 2002, and no parasitism was observed.

Larvae of Celatoria compressa (n = 34) emerged within 14.2 +/- 3.0 days (mean +/- S.D.) after collection of the hosts. Pupal period lasted 15.9 +/- 4.4 days (n = 71). Upon emergence, tachinid adults were kept in the release cage where they survived approximately 5 to 7 days (for further information see WP 3/3). No copulation of the flies or actual parasitism of the host beetles was observed. Consequently, tachinids could not yet be reared under laboratory conditions in Mexico.

All other natural enemies found during the sampling periods were of minor importance.